1. Field of The Invention
The present invention relates to a method for obtaining reflection travel times from an interpretation of seismic data in migrated cylindrical waves, for a given value of a parameter defining the slope of these waves, or from the superposition of such data associated with various substantially parallel acquisition lines.
1. Description of the Prior Art
Demigration of seismic data is the inverse of migration, which is a conventional seismic data imaging method essentially consisting, knowing the value of a wavefield at a known depth, for example at the surface, as well as a distribution model of the propagation velocities of the waves in the subsoil, in modeling the propagation of the source field and the backpropagation of the reflection data recorded, and in seeking phase coherences between these two modeled fields. Migration is particularly useful for interpretation of seismic data acquired on complex structures.
It also allows to access, via demigration, the arrival times of the reflections associated with the picked events. This operation is notably carried out for determining, by means of kinematic methods such as migration velocity analysis methods, the velocity distribution in the subsurface. This determination constitutes a key stage in complex structures imaging. An example of such a method is described by: Stork, C., 1992; <<Reflection Tomography in the Postmigrated Domain>>; Geophysics, 57, 680–692.
A particularly effective migration velocity analysis method, referred to as Smart (<<Sequential Migration Aided Reflection Tomography>>) is notably described by: Ehinger, A. and Lailly, P., 1995, Velocity Model Determination by the SMART Method, Part 1: Theory: 65th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, pp. 739–742.
Reflection tomography is used, in such methods, for determining the velocity model. The reflection travel times (reflection tomography data) are obtained, in the complex parts, by demigrating the picked events upon interpretation of the migrated data. This method has been successfully implemented for 3D imaging of complex structures by using 2D prestack migration algorithms (and the associated demigration) on a set of seismic profiles extracted from a 3D acquisition. However, such an approach is efficient only in a very specific context:                on the geologic plane, the structure must vary relatively slowly in a direction (referred to as <<strike>> in English literature);        the seismic profiles must be acquired in the orthogonal direction (therefore the complexity direction) referred to as <<dip>> in the English literature.        
On the other hand, this approach is difficult to use with complex structures without a privileged direction: such structures require prestack 3D migration techniques, which involve long calculation times, especially if imaging is carried out on a volume.
French Patent 2,784,195, filed by the assignee, notably describes another seismic data migration method known in the art as Alchemig. This method allows performing a 3D prestack migration of seismic events and imaging of the volumes of an underground zone, with an attractive calculation time, from a series of a number Ns of reflection survey cycles.
It comprises successive emission of elementary waveflelds, each defined by the association of a seismic signal W(t) and of a determined emission point from a series of emission points {right arrow over (S)}i with 1≦i≦NS, reception, by seismic receivers placed at positions {right arrow over (R)}iJ, of the seismic signals reflected by the zone in response to each one of these wavefields, and recording of the various signals received by each seismic receiver in form of time-dependent seismic traces dij (t). For a given velocity model,    a) a slowness vector {right arrow over (p)} (homogeneous to the inverse of a velocity) whose two components py and px can take a previously defined series of values is defined,    b) a time lag function t0({right arrow over (p)},i) is defined for a given slowness vector {right arrow over (p)} and for a given emission point {right arrow over (S)}i,    c) a time lag function t0({right arrow over (p)},i) is applied to each elementary wavefield associated with emission point {right arrow over (S)}i, and a composite wavefield is formed at the surface by spatiotemporal superposition of the various elementary wavefields to which such a time lag is applied,    d) a time lag t0({right arrow over (p)},i) is applied to each seismic trace dij (t) marked by pair (i,j) and a composite trace field is formed at the surface by spatiotemporal superposition of the various seismic traces to which such a time lag is applied,    e) a migration of the composite trace field is carried out by using as the wavefield the composite wavefield, which is done by modeling the propagation of the composite wavefield and the backpropagation of the composite trace field, and by suitably combining the two composite fields thus modeled at any point of the zone to be imaged,    F) steps (c to e) are repeated for all the values taken by components py and px of vector {right arrow over (p)}, and    g) for any fixed value of the second component px of the vector {right arrow over (p)}, the results of these various combinations are summed so as to obtain a migrated image associated with this fixed value of px, thus performing a prestack migration.
It is assumed that, even if the technique proves robust in case of violation of these hypotheses, the seismic data is constant-azimuth data (the azimuth being defined as the direction of the source-pickup bipoints) (hypothesis 1) and that acquisition is carried out by displacing the source along lines parallel to the direction defined by the azimuth (hypothesis 2).
If this is not the case, it can be remedied by first carrying out a preprocessing operation, for example according to the technique referred to as AMO (<<Azimuth Moveout>>), well-known to the man skilled in the art, to adjust the data obtained from standard marine acquisitions.
The co-ordinates system selected here is such that the x-axis is parallel to the direction of the acquisition lines and the y-axis thus represents the position of an acquisition line. Hereafter the measurement by cylindrical wave data is defined, along an acquisition line, of the seismic response of the subsurface to a cylindrical wave (wave generated by a source line and whose phase varies linearly with the abscissa along the acquisition line) whose axis coincides with the acquisition line considered. Considering this definition, the Aichemig technique allows obtaining the superposition, along the various acquisition lines, of migrated cylindrical wave data, these cylindrical waves being associated with an acquisition line and with a predetermined value of parameter px defining the slope of the cylindrical wave. The procedure consists in applying to the data linear phase shifts parametered by vector {right arrow over (p)} whose component along the profiles axis is px, and in calculating such a superposition by applying to the phase shifted data a plane-wave migration software. Postmigration stacking is obtained by summing the results obtained for the different values taken px. We suppose that, by suitable selection of the origin of the mark or of the phase shift function, the point of the surface where the phase shifts are zero has (x=0, y=0) as the co-ordinates.
It can be noted that the technique described above is one way among many to obtain superposition of migrated cylindrical wave data associated with parallel acquisition lines and with a given value of parameter px Another procedure, can for example, is described by Claerbout, J. F., 1971, Towards a Unified Theory of Reflector Mapping; in Geophysics, 36, No. 3, 467–481, in calculating the result by carrying out, for each acquisition line:                propagation of the cylindrical wave and back propagation of the cylindrical wave data, and        calculation, at any point of the space, of the cross correlation of the source field whose propagation has been modeled and of the back propagated data field, which allows obtaining, for the acquisition line considered, the migrated cylindrical wave data;        and by stacking (superposing) the migrated cylindrical wave data obtained for the various acquisition lines.        
If, for several values of parameter px, the superposition of migrated cylindrical wave data is associated with various acquisition lines, it is possible, as explained in the aforementioned patent, to control the quality of the velocity model used for the migration and also to update the velocity model by exploiting the deformations of the events when going from one cylindrical wave parameter to the next (migration velocity analysis).
For updating, according to the aforementioned Smart method (or others), the events have to be picked, for various values of parameter px, in the superposition of the migrated cylindrical wave data, a superposition corresponding to the various acquisition lines, then the picked events have to be demigrated and the model is updated by reflection tomography. Even it is required to be known how to demigrate the interpreted migrated events when interpretation has been performed on migrated cylindrical wave data or the superposition obtained for various acquisition lines.